The present invention relates to a method of setting a flow coefficient used in a flow meter for measuring a flow rate of a fluid.
A conventional flow meter will be described with reference to FIG. 21. A flow velocity measurement device 2 for measuring a flow velocity of a fluid, such as a thermal type flow sensor, is provided at a point in a fluid pipe 1 where a fluid passes therethrough. The flow velocity (Vm) obtained by the flow velocity measurement device 2 is multiplied by a cross-sectional area (S) of the fluid pipe 1 and a flow coefficient (K), so as to calculate a flow rate (Qm). The flow velocity measurement device 2 obtains the flow velocity (Vm) of the fluid by measuring the flow velocity of only a portion of the fluid in the vicinity of the flow velocity measurement device 2. Therefore, an average flow velocity for the entire area of the fluid pipe 1 needs to be calculated as follows. A reference flow rate setting section capable of setting a reference flow rate is connected to the fluid pipe 1 so as to pass a fluid at an appropriate reference flow rate through the fluid pipe 1 and obtain an average flow rate (Qa). Then, the relationship (K=Va/Vm; xe2x80x9cflow coefficientxe2x80x9d) between an average flow velocity (Va), which is calculated from the average flow rate value and the flow velocity (Vm) measured by the flow velocity measurement device is obtained. This relationship is measured for various reference flow rates so as to obtain a number of data Bets each including the flow velocity (Vm) and the flow coefficient (K) of the fluid.
Next, the flow velocity (Vm) of the fluid measured by the flow velocity measurement device 2 is multiplied by the flow coefficient (K) and the cross-sectional area (S) of the fluid pipe 1, thereby obtaining a measured flow rate (Qm). In other words, the measured flow rate (Qm) is obtained by calculating Qm=Kxc2x7Sxc2x7Vm. In FIG. 21, an arrow 3 denotes the direction of the fluid flow. FIG. 22 illustrates a relationship between the flow velocity (Vm) and the flow coefficient (K) which are obtained as described above. In FIG. 22, the horizontal axis represents the flow velocity (Vm) measured by the flow velocity measurement device, and the vertical axis represents the flow coefficient (K). For example, if the flow velocity (Vm) of the fluid measured by the flow velocity measurement device 2 is about 2 m/s, the flow coefficient (K) can be read from FIG. 22 to be about 0.89. Therefore, if the cross-sectional area (S) of the fluid pipe 1 is about 0.3xc3x9710xe2x88x923 m2, the measured flow rate (Qm) is:                     Qm        =                  xe2x80x83                ⁢                  2          xc3x97          0.89          xc3x97          0.3          xc3x97                      10                          -              3                                ⁢                      xe2x80x83                    ⁢                      m            3                    ⁢                      /                    ⁢          s                                        =                  xe2x80x83                ⁢                  0.534          xc3x97                      10                          -              3                                ⁢                      xe2x80x83                    ⁢                      m            3                    ⁢                      /                    ⁢          s                                        =                  xe2x80x83                ⁢                  1.9          ⁢                      xe2x80x83                    ⁢                      m            3                    ⁢                      /                    ⁢                      h            .                              
The conventional flow meter has the following problems. That is, using a number of sets of data (see FIG. 22) each including the flow velocity (Vm) and the flow coefficient (K) measured by the flow velocity measurement device, the flow velocity range is appropriately divided into regions by visual observation so as to set an optimal approximate line for each region which optimally approximates a group of data sets (flow coefficients) within the region, thereby obtaining a kinked line which optimally approximates the group of data sets (flow coefficients) across all regions.
It is time consuming and labor intensive to set such an optimal approximate straight line by repeatedly performing complicated calculations. Moreover, because the setting operation is based on a visual observation, it has a poor reproducibility, and the obtained optimal approximate straight line may vary each time it is set. Although the optimal flow coefficient may be approximated by a high-degree curve, a low-degree approximation such as a linear or quadric approximation is preferred when the calculation is done by a microcomputer, or the like, because of the limitations associated with the use of a microcomputer such as the calculation time and the number of significant digits.
When the type of a fluid is changed from that used when measuring the reference flow rate and setting the flow coefficient, it is necessary to re-measure the average flow rate (Qa) and the flow velocity (Vm) of the new fluid so as to re-set a new flow coefficient (K).
When the temperature of the fluid changes, the characteristics of the fluid may also change, thereby changing the flow coefficient and deteriorating the flow rate measurement precision.
The present invention has been made to solve the above-described problems and provides a method of setting a flow coefficient, including the steps of: obtaining an optimal approximate line using a number n of consecutive sets of data points (Xi, Yi) of all flow velocity data points measured by a flow velocity measurement section, and reference data stored in a reference data memory section; increasing or decreasing the number n so that the n sets of data points are all within a predetermined error Er with respect to the optimal approximate liner performing a calculation operation for setting a region by a flow coefficient calculation section; and storing an obtained flow coefficient in a flow coefficient memory section.
With such a structure, according to the flow coefficient setting method of the present invention having such a structure, it is possible to easily and automatically set a flow coefficient using a personal computer, or the like, with good reproducibility, while suppressing the flow rate value within a predetermined error.
Another method of setting a flow coefficient of the present invention includes the steps of: obtaining an optimal approximate curve using a plurality of sets of data points (Xi, Yi) of all flow velocity data points measured by a flow velocity measurement section, and reference data stored in a reference data memory section, dividing the optimal approximate curve into a number m of regions; performing a calculation operation for approximating each region with an optimal approximate straight line by a flow coefficient calculation section; and storing an obtained flow coefficient in a flow coefficient memory section.
With such a structure, even if the number of data points available is limited, it is possible to select an optimal curve so that a flow coefficient can be set with a reduced error over a wider range, in a more efficient manner and within a shorter period of time.
A flow meter of the present invention includes: a flow velocity measurement section for measuring a flow velocity of a fluid; a flow coefficient memory section for storing a flow coefficient which is set by the above-described method of setting a flow coefficient; and a flow rate calculation section for calculating a flow rate of the fluid from the measured flow velocity using the flow coefficient stored in the flow coefficient memory section.
With such a structure, it is possible to provide a flow meter with a reduced error over a wide flow rate range.
Various embodiments of the present invention will be described below.
A method of setting a flow coefficient according to one embodiment of the present invention includes the steps of: obtaining an optimal approximate line using a number n of consecutive sets of data points (Xi, Yi) of all flow velocity data points measured by a flow velocity measurement section, and reference data stored in a reference data memory section; increasing or decreasing the number n so that the n sets of data points are all within a predetermined error Er with respect to the optimal approximate line; performing a calculation operation for setting a region by a flow coefficient calculation section; and storing an obtained flow coefficient in a flow coefficient memory section.
With such a structure, according to the flow coefficient setting method of the present invention having such a structure, it is possible to easily and automatically set a flow coefficient using a personal computer, or the like, with good reproducibility, while suppressing the flow rate value within a predetermined error.
In a method of setting a flow coefficient according to one embodiment of the present invention, a linear function is used to represent the optimal approximate line if the n sets of data points (Xi, Yi) are distributed on both sides of the optimal approximate line in a middle portion of the optimal approximate line.
With such a structure, it is possible to set a flow coefficient with a simple linear function and thus to obtain a flow rate value with a reduced error by a a simple calculation.
In a method of setting a flow coefficient according to one embodiment of the present invention, a quadric function is used to represent the optimal approximate line if the n sets of data points (Xi, Yi) are distributed on one side of the optimal approximate line in a middle portion of the optimal approximate line.
With such a structure, it is possible to approximate a wider range, as compared with when using a linear function, using a curve with a reduced error.
A method of setting a flow coefficient according to one embodiment of the present invention includes the steps of: obtaining an optimal approximate curve using a plurality of sets of data points (Xi, Yi) of all flow velocity data points measured by a flow velocity measurement section, and reference data stored in a reference data memory section; dividing the optimal approximate curve into a number m of regions; performing a calculation operation for approximating each region with an optimal approximate straight line by a flow coefficient calculation section; and storing an obtained flow coefficient in a flow coefficient memory section.
With such a structure, even if the number of data points available is limited, it is possible to select an optimal curve so that a flow coefficient can be set with a reduced error over a wider range, in a more efficient manner and within a shorter period of time.
In a method of setting a flow coefficient according to one embodiment of the present invention, the optimal approximate curve is equally divided into the number m of regions along a y-axis direction.
With such a structure, it is possible to divide a data range into m regions along a y-axis direction within a shorter period of time, thereby efficiently setting a flow coefficient.
In a method of setting a flow coefficient according to one embodiment of the present invention, the optimal approximate curve is equally divided into the number m of regions along an x-axis direction.
With such a structure, it is possible to divide a data range into m regions along an x-axis direction within a shorter period of time, thereby efficiently setting a flow coefficient.
In a method of setting a flow coefficient according to one embodiment of the present invention, the optimal approximate curve is divided into the number m of regions along an x-axis direction such that a width of each region is inversely proportional to a gradient of the optimal approximate straight line for the region.
With such a structure, it is possible to divide a data range into m regions within a shorter period of time, while efficiently setting a flow coefficient so that the errors of the respective regions are close to one another.
In a method of setting a flow coefficient according to one embodiment of the present invention, the optimal approximate curve is represented by Y=axc3x97Log(X)+b.
With such a structure, it is possible to divide a setting range into m regions to linearly approximate each region with as few as two data points.
In a method of setting a flow coefficient according to one embodiment of the present invention, the optimal approximate curve is represented by Y=(axe2x88x92b)/[1+exp(xe2x88x92cxc3x97X)]+b.
With such a structure, it is possible to divide a wide setting range into n regions to linearly approximate each region with a small number of data points.
In a method of setting a flow coefficient according to one embodiment of the present invention, the flow velocity measurement section includes a thermal type flow sensor.
With such a structure, it is possible to set a flow coefficient with a reduced error and a good reproducibility particularly in a low flow rate region.
In a method of setting a flow coefficient according to one embodiment of the present invention, the flow velocity measurement section includes an ultrasonic flow meter.
With such a structure, it is possible to set a flow coefficient with a reduced error and a good reproducibility over a wide flow rate range.
In a method of setting a flow coefficient according to one embodiment of the present invention, the optimal approximate line is represented by a low-degree function which is a linear function or a quadric function.
With such a structure, it is possible to obtain a flow rate value with a reduced error by a simple calculation.
In a method of setting a flow coefficient according to one embodiment of the present invention, a data point which is included by two adjacent regions is set to belong to one of the two adjacent regions in which an error Er calculated based on the optimal approximate line is smaller.
With such a structure, it is possible to reduce the error for a boundary value.
In a method of setting a flow coefficient according to one embodiment of the present invention, an intersection between two optimal approximate lines for two adjacent regions is used as a boundary point between the two regions.
With such a structure, it is possible to smoothly connect the region boundary points to one another.
In a method of setting a flow coefficient according to one embodiment of the present invention, the error Er is gradually increased until an entire data range required can be divided into a predetermined number of regions.
With such a structure, even when the number of regions is prescribed, it is possible to divide a data range into the prescribed number of regions while setting a flow coefficient with a minimum error.
In a method of setting a flow coefficient according to one embodiment of the present invention, when a type of a fluid changes from a first fluid to a second fluid, an x-axis value of a flow coefficient is multiplied by a fluid-type-dependent constant so as to convert the flow coefficient to a new flow coefficient.
With such a structure, even when the type of a fluid changes from that used when setting a flow coefficient, the flow coefficient can easily be converted to a new flow coefficient for the new fluid, thereby suppressing an error which may be caused by such a change in the type of a fluid.
In a method of setting a flow coefficient according to one embodiment of the present invention, the constant is a new flow velocity (Vmxc3x97Vg/Vm) which is obtained by multiplying a flow velocity ratio (Vg/Vm) to a flow velocity (Vm) of the first fluid, where Vg is a flow velocity of the second fluid for any flow coefficient value (Kc).
With such a structure, even when there is a change in the type of a fluid, it is possible to update the flow coefficient using only one data point according to the type a of the fluid, thereby eliminating the need to re-measure the flow coefficient.
In a method of setting a flow coefficient according to one embodiment of the present invention, when a temperature of a fluid changes from a first temperature to a second temperature, an x-axis value of a flow coefficient is multiplied by a temperature-dependent function value so as to convert the flow coefficient to a new flow coefficient.
With such a structure, even when the temperature of the fluid changes from that when setting a flow coefficient, the flow coefficient can easily be converted to a new flow coefficient for the new temperature, thereby suppressing an error which may be caused by such a change in the temperature of the fluid.
In a method of setting a flow coefficient according to one embodiment of the present invention, the function value used for obtaining the new flow coefficient is calculated by the following expression:
Vi(Ts/Ti)p
where Ts denotes the first temperature, Ti denotes the second temperature, Vi denotes a flow velocity of the fluid measured at Ti, and p denotes an exponent.
With such a structure, even when there is a change in the temperature of a fluid, it is possible to obtain a flow coefficient for the new temperature, thereby suppressing an error which may be caused by such a change in the temperature of the fluid.
In a method of setting a flow coefficient according to one embodiment of the present invention, an absolute temperature (Tm) of the fluid is determined from a temperature-sensitive resistor of a thermal type flow sensor.
With such a structure, it is not necessary to separately provide a temperature sensor, thereby realizing an efficient setting method.
In a method of setting a flow coefficient according to one embodiment of the present invention, an absolute temperature (Tm) of the fluid is determined from an ultrasonic wave propagation time from an ultrasonic flow meter.
With such a structure, it is not necessary to separately provide a temperature sensor, while realizing an accurate hydraulic temperature measurement utilizing the characteristics of a fluid.
A flow meter according to one embodiment of the present invention includes: a flow velocity measurement section for measuring a flow velocity of a fluid; a flow coefficient memory section for storing a flow coefficient which is set by the above-described method of setting a flow coefficient; and a flow rate calculation section for calculating a flow rate of the fluid from the measured flow velocity using the flow coefficient stored in the flow coefficient memory section.
With such a structure, it is possible to provide a flow meter with a reduced error over a wide flow rate range.
In a flow meter according to one embodiment of the present invention, the flow velocity measurement section includes a thermal type flow sensor.
With such a structure, it is possible to provide a flow meter with a reduced error and with good reproducibility particularly in a low flow rate region.
In a flow meter according to one embodiment of the present invention, the flow velocity measurement section includes an ultrasonic flow meter.
With such a structure, it is possible to provide a flow meter with a reduced error and with good reproducibility over a wide flow rate range.